1. Field
The present embodiments generally relate to a tire information detecting system and, in particular, to a tire information detecting system used for motor vehicles to detect tire information, such as a tire pressure.
2. Related Art
Wireless transmission systems have been developed that wirelessly transmit a measured value, such as tire pressure of a motor vehicle or the like, to a controller disposed on the body of the motor vehicle in order to evaluate that value for outputting a warning message to a driver (refer to, for example, Japanese Patent No. 3494440 and, in particular, FIGS. 3 and 5). Such wireless transmission systems include a controller, as shown in FIG. 6, disposed on the body of a motor vehicle and a measured value transmitter (transponder), as shown in FIG. 7, disposed in a tire.
As shown in FIG. 6, the controller includes a carrier wave oscillator G1 for generating carrier waves f1 having a frequency of about 2.4 GHz, a modulator MO1, and an oscillator G2 for outputting an oscillation signal for modulation. The oscillator G2 outputs, to the modulator MO1, an oscillation signal having a frequency f2 that is close to the resonance frequency of a resonator of a transponder, which is described below. The carrier waves output from the carrier wave oscillator G1 are amplitude-modulated by the oscillation signal output from the oscillator G2. Subsequently, the amplitude-modulated 2.4-GHz high-frequency signal is amplified by an amplifier (not shown) and is emitted from an antenna A1 disposed in the vicinity of the tire.
The controller further includes a switch S1, a receiver E1, and a timer T1. The switch S1 is used for selecting whether or not the amplitude modulation is performed by the modulator MO1. The receiver E1 receives a high-frequency signal emitted from the transponder and computes a measured value (S1), such as a tire pressure. The timer T1 controls the switching timing of the switch S1 and the state of the receiver E1. After the timer T1 sets the carrier waves to be amplitude-modulated so that an amplitude-modulated high-frequency signal is transmitted for a predetermined time period, the amplitude modulation is stopped at a time t1. Thereafter, unmodulated carrier waves are transmitted. The receiver E1 becomes active at a time t2 which is within about 1 μs from the time t1 so as to receive the high-frequency signal output from the transponder via an antenna A4.
As shown in FIG. 7, the transponder includes a low-pass filter L1/C1, a diode D1 serving as a modem, and a capacitive pressure sensor (hereinafter simply referred to as a “pressure sensor”) SC1 whose capacitance varies in accordance with a tire pressure, and a resonator including a quartz crystal resonating element Q1 that is excited by a frequency component included in the high-frequency signal output from the controller. The 2.4-GHz carrier waves are removed from the high-frequency signal output from the controller by the low-pass filter L1/C1.
The high-frequency signal is demodulated by the diode D1. A signal having a frequency that is the same as that of the oscillation signal from the oscillator G2 is retrieved. Since the resonator has a resonance frequency close to the frequency of the oscillation signal from the oscillator G2, the resonator is excited by the signal generated here. This excitation generates a signal having the resonance frequency. Note that since the resonance frequency of the resonator varies as the capacitance of the pressure sensor SC1 varies in accordance with the tire pressure, the signal of the resonance frequency generated here is effected by the variation.
The controller transmits the amplitude-modulated high-frequency signal and, subsequently, the controller stops the amplitude modulation so as to continuously transmit unmodulated carrier waves. After the amplitude modulation is stopped, the resonator still oscillates for at least about 1 ms. The unmodulated carrier waves output from the controller are amplitude-modulated by the diode D1 in accordance with a signal having the resonance frequency of the resonator and are emitted from an antenna A3. The receiver E1 receives the amplitude-modulated high-frequency signal via the antenna A4 and retrieves the signal having the resonance frequency using, for example, a modem (not shown). In this way, the controller can compute the measured value (V1), such as a tire pressure.
In the wireless transmission system described in Japanese Patent No. 3494440, the transponder can include a reference resonator including a quartz crystal resonating element resonator and additional resonators. Thus, the transponder transmits measured values, such as the tire temperature and the structural stress of the tire, so that the controller can compute these measured values.
However, in the above-described wireless transmission systems, the resonance frequency of the resonator in the transponder is effected by the desired tire information such as a tire pressure and also another factor such as temperature. Thus, an error occurs in the measured value due to a change in temperature in the tire, and therefore, an accurate measured value cannot be detected.
In addition, even when the transponder further includes a reference resonator to compute the measured value of a tire pressure, an error in the measured value still occurs because the temperature characteristics and the secular change characteristics of the quartz crystal resonating elements of the resonators are different. Accordingly, an accurate measured value cannot be detected.